Temperature measurement and monitoring in electric machines (for example, generators, motors, transforms and the like) often necessitate special precautions due to the presence of electromagnetic fields and/or mechanical vibrations. In particular, the presence of metallic or conductive parts in the part of the temperature sensing head which is inside or near those machines should be avoided, especially when monitoring the temperature of stator bars of generators and/or motors or the temperature of the transformer coils. Also, metallic or conductive components can create partial discharge by changing the path of the electrical field.
In addition, temperature monitoring in such electromagnetic environments necessitates sensors that are reliable over a long period of time to avoid any kind of false alarms that can be costly. Further, these temperature sensors should be rugged and vibration-proof since such sensors are often expected to operate under vibratory conditions.
For such applications, temperature to be measured are generally less than 200° C. Since numerous sensors often may have to be located in various parts of a machine, monitoring costs, such as installation, measurement, maintenance and repair costs should be minimized and such sensors should be as small and as minimally intrusive as possible.
Temperature sensors that make use of fiber optics may transform temperature variations into light variations that can then be analyzed by photoelectric and/or electronic means far from the sensor head and from the electromagnetic environment. Examples of known temperature sensors incorporating fiber optics are described below.
U.S. Pat. No. 5,031,987 to Norling, which is incorporated herein by reference, describes an optical transducer facing the angled end of a single light emitting and receiving optical fiber so that any movement of the transducer (due to a temperature or pressure change) modifies the light reflected back into the optical fiber. To reduce the sensitivity of the system to shock and vibration, Norling discloses the use of a magnetically latched bimetallic thermal strip as a sensor element and a magnet that engages one end of the sensor element. In operation, the bimetallic strip, as a function of temperature, responds to magnetic attraction forces caused by the magnet.
U.S. Pat. No. 5,295,206 to Mishenko, which is incorporated herein by reference, describes a temperature sensor for the human body where a small air gap is increased or decreased by the relative dilation or retraction of a temperature sensitive rod fitted inside a metallic cylinder having a different temperature expansion coefficient. A reflective surface at one end of the rod reflects incoming light emitted through an optical fiber toward a parallel, and closely positioned, light receiving optical fiber. Change of received reflected light represents the change in temperature. However, little detail is supplied on the precise way in which the reflected light reaches the receiving optical fiber. Specifically, in optical fiber sensors, a non-negligible amount of reflected light may travel all around the air volume separating the emitting and the receiving fibers and may cause a lot of “noise”, thus affecting the temperature change sensitivity and reproducibility of the temperature measurements. Moreover, movement of the optical fibers may cause non-reproducibility of measurements. In addition, minimum lateral friction of the rod over a long time period and under any temperature and vibration conditions must be assured, which would require great mechanical precision and fine adjustment costs.
U.S. Pat. No. 5,870,511 to Sawatari et al., which is incorporated herein by reference, uses a similar variable air gap principle as U.S. Pat. No. 5,295,206 to Mishenko. In Sawatari, a sensor head has a sensor housing coupled to the end of one optical fiber. A metallic reflective surface is coupled to the housing adjacent to the end of the optical fiber to form a gap having a predetermined length between the reflective surface and the optical fiber. A detection system is also coupled to the optical fiber which determines the temperature at the sensor head from an interference pattern of light which is reflected from the reflective surface. In addition to the issues discussed herein with respect to the device disclosed in the Mishenko reference, substantial costs for the analysis of the interference patterns in Sawatari must be taken into account.
U.S. Pat. No. 5,359,445 to Robertson, which is incorporated herein by reference, describes a temperature sensor with a cylindrical housing that dilates or retracts radially in connection with external temperature changes. The housing contains two opposing optical fibers separated by a transparent, flexible and patterned film that is sealed to the housing and which deforms with the housing's movement. This sensor could be used to measure the temperature of the gas or fluid in which the sensor is immerged but does not appear to lend itself to the measurement of the temperature of a solid since its cylindrical housing's movement and the film's pattern deformation could be hindered or at least biased by the friction of its cylindrical base against the solid. Also, no provision is made against the effects of vibrations having a radial component.
Other patents (for example, U.S. Pat. No. 6,960,019 to Dammann and U.S. Pat. Nos. 5,392,117 and 5,202,939, both to Belleville, et al., all of which are incorporated herein by reference) disclose the analysis of light interference patterns and Fabry-Pérot interferometry to propose small fiber optic temperature sensors. Such sensors, however, are generally required to be used in conjunction with complex, delicate and costly interference analysis equipment.
Accordingly, it would be desirable to provide a small and rugged fiber optic temperature sensor that may be effectively used in electromagnetic and/or vibratory environments to measure the temperature of a given part of a machine or apparatus and which does not require complex and costly equipment to analyze the information coming from the sensor.